I'm not sure it's the compiler, here's the point where it fails, first is my try, second is the try I found on a clean installed karmic on the efikamx:

set -ex; \                                                                                 
        for file in ../include/mysql/plugin.h ../include/mysql.h; do \                     
                 gcc -E -nostdinc -dI \                                                    
                          -I../include \                                                   
                          -I../include/mysql \                                             
                          -I../sql \                                                       
                          -I./include \                                                    
                          -I./include/mysql \                                              
                          -I./sql \                                                        
                                         $file 2>/dev/null | \                             
                          /bin/sed -e '/^# /d' \                                           
                                    -e '/^[     ]*$/d' \                                   
                                    -e '/^#pragma GCC set_debug_pwd/d' \                   
                                    -e '/^#ident/d' > \                                    
                                               ./abi_check.out; \                          
                          /usr/bin/diff -w $file.pp ./abi_check.out; \                     
                          /bin/rm ./abi_check.out; \                                       
        done                                                                               
+ for file in ../include/mysql/plugin.h ../include/mysql.h                                 
+ /bin/sed -e '/^# /d' -e '/^[  ]*$/d' -e '/^#pragma GCC set_debug_pwd/d' -e '/^#ident/d'  
+ gcc -E -nostdinc -dI -I../include -I../include/mysql -I../sql -I./include -I./include/mysql -I./sql ../include/mysql/plugin.h
+ /usr/bin/diff -w ../include/mysql/plugin.h.pp ./abi_check.out                                                                
+ /bin/rm ./abi_check.out                                                                                                      
+ for file in ../include/mysql/plugin.h ../include/mysql.h                                                                     
+ gcc -E -nostdinc -dI -I../include -I../include/mysql -I../sql -I./include -I./include/mysql -I./sql ../include/mysql.h       
+ /bin/sed -e '/^# /d' -e '/^[  ]*$/d' -e '/^#pragma GCC set_debug_pwd/d' -e '/^#ident/d'                                      
+ /usr/bin/diff -w ../include/mysql.h.pp ./abi_check.out                                                                       
2,674d1                                                                                                                        
< typedef char my_bool;                                                                                                        
< typedef int my_socket;                                                                                                       
< #include "mysql_version.h"                                                                                                   
< #include "mysql_com.h"                                                                                                       
< enum enum_server_command

..........


set -ex; \                                                                           
        for file in ../include/mysql/plugin.h ../include/mysql.h; do \               
                 gcc -E -nostdinc -dI \                                              
                          -I../include \                                             
                          -I../include/mysql \                                       
                          -I../sql \                                                 
                          -I./include \                                              
                          -I./include/mysql \                                        
                          -I./sql \                                                  
                                         $file 2>/dev/null | \                       
                          /bin/sed -e '/^# /d' \                                     
                                    -e '/^[     ]*$/d' \                             
                                    -e '/^#pragma GCC set_debug_pwd/d' \             
                                    -e '/^#ident/d' > \                              
                                               ./abi_check.out; \                    
                          /usr/bin/diff -w $file.pp ./abi_check.out; \               
                          /bin/rm ./abi_check.out; \                                 
        done                                                                         
+ for file in ../include/mysql/plugin.h ../include/mysql.h                           
+ gcc -E -nostdinc -dI -I../include -I../include/mysql -I../sql -I./include -I./include/mysql -I./sql ../include/mysql/plugin.h
+ /bin/sed -e '/^# /d' -e '/^[  ]*$/d' -e '/^#pragma GCC set_debug_pwd/d' -e '/^#ident/d'                                      
+ /usr/bin/diff -w ../include/mysql/plugin.h.pp ./abi_check.out                                                                
+ /bin/rm ./abi_check.out                                                                                                      
+ for file in ../include/mysql/plugin.h ../include/mysql.h                                                                     
+ gcc -E -nostdinc -dI -I../include -I../include/mysql -I../sql -I./include -I./include/mysql -I./sql ../include/mysql.h       
+ /bin/sed -e '/^# /d' -e '/^[  ]*$/d' -e '/^#pragma GCC set_debug_pwd/d' -e '/^#ident/d'                                      
+ /usr/bin/diff -w ../include/mysql.h.pp ./abi_check.out                                                                       
+ /bin/rm ./abi_check.out                                                                                                      
make[3]: Leaving directory `/home/markos/mysql-dfsg-5.1-5.1.37/builddir'                                                       
make[2]: Leaving directory `/home/markos/mysql-dfsg-5.1-5.1.37/builddir'                                                       
Making all in include                                                                                                          
make[2]: Entering directory `/home/markos/mysql-dfsg-5.1-5.1.37/builddir/include'                                              
/usr/bin/make  all-am

................

See the difference? himt, this is what breaks the build:
(mine)
+ for file in ../include/mysql/plugin.h ../include/mysql.h                                 
+ /bin/sed -e '/^# /d' -e '/^[  ]*$/d' -e '/^#pragma GCC set_debug_pwd/d' -e '/^#ident/d'  
+ gcc -E -nostdinc -dI -I../include -I../include/mysql -I../sql -I./include -I./include/mysql -I./sql ../include/mysql/plugin.h

(original)
+ for file in ../include/mysql/plugin.h ../include/mysql.h                           
+ gcc -E -nostdinc -dI -I../include -I../include/mysql -I../sql -I./include -I./include/mysql -I./sql ../include/mysql/plugin.h
+ /bin/sed -e '/^# /d' -e '/^[  ]*$/d' -e '/^#pragma GCC set_debug_pwd/d' -e '/^#ident/d'          

I'll look into it a bit more.